--files doesn't correctly handle .gitignore'd directories when supplied paths are absolute

[nickstenning]

It seems that specifying a path in a .gitignore which has a trailing slash, such as

a/

(as opposed to simply a) causes rg --files to fail to correctly ignore files within that directory. This only happens when an absolute path is supplied as an argument.

Steps to reproduce

Here's a shell session that shows the problem. In it I create a directory, ignore it, and then show that the files within that directory are correctly ignored if I run rg --files or rg --files ignored . (see #64 for why I've included the word ignored in that invocation), but not if I run rg --files ignored $PWD.

If I either specify the path as ., or remove the trailing slash from the line in .gitignore, the directory is correctly ignored.

$ uname -mrsv
Darwin 15.6.0 Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64 x86_64
$ rg --version
0.1.17
$ mkdir a
$ touch x y a/x a/y
$ echo a/ > .gitignore
$ ag . -l -g ''
x
y
$ ag $PWD -l -g ''
/Users/redacted/mess/current/example/x
/Users/redacted/mess/current/example/y
$ rg --files ignored .
./x
./y
$ rg --files ignored $PWD
/Users/redacted/mess/current/example/a/x
/Users/redacted/mess/current/example/a/y
/Users/redacted/mess/current/example/x
/Users/redacted/mess/current/example/y
$ echo a > .gitignore
$ rg --files ignored .
./x
./y
$ rg --files ignored $PWD
/Users/redacted/mess/current/example/x
/Users/redacted/mess/current/example/y

[BurntSushi]

Nice find. This is a dupe of #16.

[BurntSushi]

(Fix incoming.)